This invention relates to power distribution overcurrent protection. More specifically it relates to a method for generating a circuit interruption signal based on a generalized time delay function during which time delay a current overload is allowed to continue. Further, it relates to inherent thermal protection which may be added to and included with the generalized function to increase the versatility and response time of the afforded protection.
Circuit protection means, such as circuit breakers, contactors, or fuses, in industrial power distribution systems or networks are used to provide protection against faults such as overloads and short circuits. Faults generally cause current to flow through more than one circuit breaker. A heirarchy is generally predeterminedly established to trip the desired circuit breaker, i.e. preferably the one electrically closest to the fault which will serve to isolate the fault while permitting the rest of the network to continue normal operation. This heirarchy is typically achieved by coordination of breaker response to time versus current detection characteristics at each circuit breaker. These characteristics are typically selected so that the desired breaker trips to remove the fault before sufficient time elapses for any other breaker to react. Thus, the branches of the network not experiencing a fault condition are permitted to continue normal operation.
In a typical industrial power distribution system, the electrical current must pass through several levels of protective breakers before it reaches the ultimate load. When it is desired to coordinate the control of the individual breakers during short circuits and overloads, a decision is generally made at each breaker based on a time versus current curve. For values of current greater than a preset pickup level, the curve indicates how long the breaker will delay before it trips or opens the circuit. Tripping strategies must be selected at each breaker to insure that only the breaker electrically near the fault or overload is tripped and, at the same time, to insure that the loads and power system conductors are protected from damage.
Two methods which are currently used to implement time versus current detection characteristics to provide circuit protection employ either thermal-magnetic devices or special purpose electronic timers.
A thermal-magnetic device is generally able to account for time varying loads to provide inherent thermal protection. Thermal protection is generally achieved by passing the breaker current through resistance heaters. For example, an overload subsequent to a continuous current at the rated value is detected more rapidly than an overload that occurs during initial energization or one that occurs after a continuous current current at less than the rated value. A disadvantage of thermal-magnetic protection is the relative inflexibility of the shape of its time versus current detection curve. The shape of the curve follows naturally from the thermal-magnetic principle and this places limitations on the number of breakers which can be coordinated since the shape cannot be readily changed.
Special purpose electronic timers are used to produce time versus current tripping characteristics that are generally more flexible than those of thermal-magnetic devices. However, they do not provide the inherent thermal protection of a thermal-magnetic device. That is, the time required to detect an overload does not depend on the current conditions prior to the overload. An overload on an initially fully loaded circuit, i.e. current at the rated value, is detected after the same time delay that occurs for detection of an overload on an initially unloaded circuit, i.e. no current or initial energization.